viruses
CHAPTER SUMMARY 21.1 Viral Evolution, Morphology, and Classification Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNAnever bothand they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.